This invention pertains, inter alia, to microlithography, which involves the transfer of a pattern, usually defined by a reticle or mask, onto a xe2x80x9csensitivexe2x80x9d substrate using an energy beam. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, thin-film magnetic pickup heads, and micromachines. More specifically, the invention pertains to methods and devices, used in the context of a microlithography method and apparatus, respectively, for rotating a substrate as required for assessing undesired measurement error of the substrate position.
As the density and miniaturization of microelectronic devices have continued to increase, the accuracy and resolution demands imposed on microlithographic methods also have increased. Currently, most microlithography is performed using, as an energy beam, a light beam (typically deep-UV light) produced by a high-pressure mercury lamp or excimer laser, for example. These microlithography apparatus are termed xe2x80x9copticalxe2x80x9d microlithography apparatus. Emerging microlithographic technologies include charged-particle-beam (xe2x80x9cCPBxe2x80x9d; e.g., electron-beam) microlithography and xe2x80x9csoft-X-rayxe2x80x9d (or xe2x80x9cextreme UVxe2x80x9d) microlithography. Because many contemporary microlithography machines operate according to the well-known xe2x80x9cstep-and-repeatxe2x80x9d exposure scheme, they are often referred to generally as xe2x80x9csteppers.xe2x80x9d
All microlithographic technologies involve pattern transfer to a suitable substrate, which can be, for example, a semiconductor wafer (e.g., silicon wafer), glass plate, or the like. So as to be imprintable with the pattern, the substrate typically is coated with a xe2x80x9cresistxe2x80x9d that is sensitive to exposure, in an image-forming way, by the energy beam in a manner analogous to a photographic exposure. Hence, a substrate prepared for microlithographic exposure is termed a xe2x80x9csensitivexe2x80x9d substrate.
For microlithographic exposure, the substrate (also termed herein a xe2x80x9cwaferxe2x80x9d) typically is mounted on a substrate stage (also called a xe2x80x9cwafer stagexe2x80x9d). The wafer stage is a complex and usually quite massive device that not only holds the wafer for exposure (with the resist facing in the upstream direction) but also provides for controlled movement of the wafer in the X- and Y-directions (and sometimes the Z-direction) as required for exposure and for alignment purposes. In most microlithography apparatus, a number of devices are mounted to and supported by the wafer stage. These devices include a xe2x80x9cwafer tablexe2x80x9d and a xe2x80x9cwafer chuckxe2x80x9d attached to the wafer table. The wafer table can be used to perform fine positional adjustment of the wafer relative to the wafer stage, and often is configured to perform limited tilting of the wafer chuck (holding the wafer) relative to the Z-axis (e.g., optical axis).
The wafer chuck is configured to hold the wafer firmly for exposure and to facilitate presenting a planar sensitive surface of the wafer for exposure. The wafer usually is held to the surface of the wafer chuck by vacuum, although other techniques such as electrostatic attraction also are employed under certain conditions. The wafer chuck also facilitates the conduction of heat away from the wafer that otherwise may accumulate in the wafer during exposure.
Monitoring of the position of the wafer in the X-, Y-, and Z-directions must be performed with extremely high accuracy to ensure the attainment of the desired accuracy of exposure of the pattern from the reticle to the wafer. The key technology employed for such purposes is interferometry, due to the extremely high accuracy obtainable with this technology. Interferometry usually involves the reflection of light from mirrors, typically located on the wafer table, and the generation of interference fringes that are detected. Changes in the pattern of interference fringes are detected and interpreted as corresponding changes in position of the wafer table (and thus the wafer). To facilitate measurements in both the X- and Y-directions over respective ranges sufficiently broad to encompass the entire wafer, the wafer table typically has mounted thereto an X-direction movable mirror and a Y-direction movable mirror. The X-direction movable mirror usually extends in the Y-direction along a full respective side of the wafer table, and the Y-direction movable mirror usually extends in the X-direction along a full respective side of the wafer table.
Despite the extremely high accuracy with which modern microlithography apparatus are constructed and with which positional measurements can be performed in these apparatus, the measurements still are not perfect and hence are characterized by certain tolerances. With respect to these tolerances, a measurement error caused by the apparatus itself is termed a xe2x80x9ctool-induced shift,xe2x80x9d or xe2x80x9cTIS,xe2x80x9d an error attributed to variations in the wafers (or other substrates) is termed a xe2x80x9cwafer-induced shift,xe2x80x9d or xe2x80x9cWIS.xe2x80x9d The term xe2x80x9ctoolxe2x80x9d is derived from the common reference to a microlithography apparatus as a xe2x80x9clithography tool.xe2x80x9d
Whenever a wafer is mounted on the wafer chuck, the microlithography apparatus normally executes an alignment routine to determine the precise position and orientation of the wafer before initiating exposure of the wafer. To such end, the wafer chuck typically includes xe2x80x9cfiducialxe2x80x9d (reference) marks strategically placed around the wafer. Similarly, the wafer itself typically includes multiple alignment marks imprinted thereon.
Reference now is made to FIG. 6, depicting a schematic plan view of a conventional stepper machine S in the region of the wafer stage WS. The wafer stage WS includes a wafer table WT and a wafer chuck WC. The wafer table WT includes an X-direction movable mirror MX and a Y-direction movable mirror My. In the stepper S, the wafer stage WS is movable (to the left and right in the figure) to assume either of two positions, an alignment position PA and an exposure position PE. At the alignment position PA, the wafer table WT is positioned relative to an alignment axis AA extending in the Z-axis direction in the figure. At the exposure position PE, the wafer table WT is positioned relative to an exposure axis AE, also extending in the Z-axis direction parallel to the alignment axis AA. The alignment axis AA is coincident with the optical axis of an alignment microscope (not shown, but situated above the plane of the page of the figure). The exposure axis AE is coincident with the optical axis of a projection-optical system (not shown but situated above the plane of the page of the figure).
Whenever the wafer stage WS is in a loading position near the alignment position PA, a wafer W can be conveyed (usually robotically) into the stepper S and placed on the wafer chuck WC on the wafer table WT. Subsequently, the wafer stage WS moves to the alignment position PA, at which the alignment microscope is used to align the wafer W on the wafer chuck WC and perform other pre-exposure alignments of the wafer as required. (To such end, the wafer W can include alignment marks M, discussed below.) Upon completion of measurements and alignments performed at the alignment position PA, the wafer stage WS moves (note arrow AR) the wafer table WT (with wafer chuck WC and wafer W) to the exposure position PE. At the exposure position PE, further measurements and alignments of the wafer table WT usually are performed. Also, if conditions are appropriate, the wafer W is exposed with a pattern defined by a reticle (not shown but situated on the exposure axis AE above the plane of the figure).
As alignments of the wafer W are being performed with the wafer stage WS at the alignment position PA, the respective positions of the wafer table WT in the X-direction and the Y-direction are monitored and determined by respective interferometers IFXL, IFYA that direct respective laser light beams at the respective movable mirrors MX, MY. Similarly, whenever the wafer stage WS is at the exposure position PE, the respective positions of the wafer table WT in the X-direction and the Y-direction are monitored and determined by respective interferometers IFXL and IFYP. Note that the X-direction interferometer IFXL is used to monitor position of the wafer table WT in both the alignment position PA and the exposure position PE.
The interferometers IFXL, IFYP, IFYA are connected to a controller (also termed a xe2x80x9cprocessorxe2x80x9d or xe2x80x9ccomputerxe2x80x9d) C. As is well known, the controller C is configured to receive data from the interferometers and to perform arithmetical calculations by which data from the interferometers are converted to data concerning the X-Y position of the wafer table WT. Typically, the controller C also is connected to any of various other data-producing and data-responsive components of the stepper machine S, and thus serves to coordinate and execute overall operation of the stepper machine S.
Normally, the optical axis AA of the alignment microscope is oriented extremely accurately parallel to the optical axis AE of the projection-optical system of the stepper S. This ensures that an accurate measurement of the wafer W by the alignment microscope results in the wafer W being sufficiently accurately positioned and aligned for actual microlithographic exposure. However, because of variations in the photoresist or wafer topology, certain errors can become manifest in the alignment or mark-position results obtained using the alignment microscope. As a result, for example, the apparent position of an alignment mark M as determined by the alignment microscope is actually displaced from the actual (xe2x80x9crealxe2x80x9d) position of the mark M. This displacement is an example of the TIS to which the present invention is directed.
In view of the shortcomings of conventional apparatus as summarized above, a first aspect of the invention is set forth in the context of microlithography methods performed using a microlithography apparatus including a wafer stage and a xe2x80x9cholding member,xe2x80x9d and provides methods for measuring a tool-induced shift. As used herein, a xe2x80x9cholding memberxe2x80x9d is any suitable member configured to hold a substrate relative to the wafer stage. For example, the holding member can include a wafer table and a wafer chuck to which the substrate is mounted.
In a first embodiment of a method according to the invention, the holding member is provided with rotatability, relative to the wafer stage, from a first rotational position to a second rotational position. At the first rotational position, a respective location of an alignment feature on the holding member is determined. The holding member then is rotated about a rotational axis to the second rotational position. At the second rotational position, a respective location of the alignment feature is determined. The respective location of the alignment feature determined at the first rotational position is compared with the respective location of the alignment feature at the second rotational position. A corresponding tool-induced shift is determined from a detected difference between the respective locations.
As noted above, the alignment feature (e.g., an alignment mark) is located xe2x80x9conxe2x80x9d the holding member. As this expression is used herein, the alignment feature can be located, for example, on the substrate (e.g., semiconductor wafer) or on a wafer chuck to which the substrate is mounted. Further alternatively, in some embodiments, the alignment feature can be located on the wafer table if the holding member includes a wafer table, wherein a wafer chuck is mounted to the wafer table.
For rotatability of the holding member, a rotary actuator is provided that is situated and configured to rotate the holding member relative to the wafer stage about the rotational axis from the first rotational position to the second rotational position. The rotary actuator can be any of various suitable devices for imparting a controlled rotation of the holding member, about the rotational axis, over a defined angular displacement, such as 90xc2x0 and/or 180xc2x0, relative to the wafer stage. At each of the first and second rotational positions, the alignment feature can be imaged, to facilitate alignment of the alignment feature with the alignment reference, using a microscope. Meanwhile, coordinates of the alignment feature are determined. Typically, the coordinates are in an X-Y plane perpendicular to an optical axis of an imaging system.
The wafer stage can be movable back and forth between an alignment position and an exposure position. In such an instance, the respective determinations of the respective locations of the alignment feature can be made at the alignment position or at both the alignment position and the exposure position.
In another embodiment of a method according to the invention for measuring tool-induced shift, the wafer table includes a wafer chuck to which a substrate is mounted. A rotary actuator is provided that is situated and configured to rotate the wafer table and wafer chuck relative to the wafer stage about a rotational axis from a first rotational position to a second rotational position. An alignment feature is provided on at least one of the wafer table, wafer chuck, and substrate. At the first rotational position, the alignment feature is aligned with an alignment reference, and a respective positional coordinate of the wafer table is determined. At the second rotational position, the alignment feature is aligned with the alignment reference, and a respective positional coordinate of the wafer table is determined. The respective coordinate determined at the first rotational position is compared with the respective coordinate determined at the second rotational position, and a corresponding tool-induced shift is determined from a detected difference between the respective coordinates.
As noted above, the first and second rotational positions can be, for example, 90xc2x0 and/or 180xc2x0 relative to each other. At each rotational position, the respective alignments desirably are performed using a microscope, and the respective positional coordinates desirably are determined using respective interferometers, which can be the same interferometers or one or more different interferometers as required at each rotational position.
The wafer stage can be movable back and forth between an alignment position and an exposure position. At the alignment position or at both the alignment position and the exposure position, an interferometer(s) produces data on the positional coordinates of the holding member.
Another aspect of the invention, set forth in the context of microlithography apparatus that include a wafer table and a wafer stage, provides devices for measuring a tool-induced shift. An embodiment of such a device comprises a rotary actuator that is situated and configured to rotate the wafer table relative to the wafer stage from a first rotational position to a second rotational position. The device also includes an alignment-measurement device situated and configured to align an alignment feature, carried by the wafer table, with an alignment reference. At least one respective interferometer is provided for the first and second rotational positions. The respective interferometers are situated and configured to obtain data on positional coordinates of the wafer table at the first and second rotational positions as the alignment feature is aligned with the alignment reference. A controller is connected to the rotary actuator and the interferometers. The controller is configured to determine a tool-induced shift from the positional coordinates at each of the first and second rotational positions of the wafer table.
As noted above, the rotary actuator can be configured to rotate the wafer table 90xc2x0 and/or 180xc2x0 relative to the wafer stage from the first rotational position to the second rotational position. Also, the wafer stage can be movable back and forth between an alignment position and an exposure position.
Also as noted above, the alignment-measurement device desirably comprises an alignment microscope, and the wafer table desirably includes a wafer chuck mounted to the wafer table. The wafer table desirably comprises an X-direction movable mirror and a Y-direction movable mirror, wherein the apparatus further comprises one or more respective interferometers for each of the X-direction movable mirror and Y-direction movable mirror at each of the first and second rotational positions. The number of movable mirrors is not limited to one in each of the X- and Y-directions. Multiple movable mirrors in either or both directions can be employed, which can reduce the number of interferometers required.
In another embodiment of a device according to the invention, a rotary actuator is situated and configured to rotate the wafer table relative to the wafer stage about a rotational axis from a first rotational position to a second rotational position. An alignment microscope is situated and configured to align an alignment feature, carried by the wafer table, with an alignment reference. At least one respective interferometer is situated and configured to obtain data on positional coordinates of the wafer table at each of the first and second rotational positions as the alignment feature is aligned with the alignment reference. The device also includes a controller, connected to the rotary actuator and the interferometers, that is configured to determine a tool-induced shift from the positional coordinates at each of the first and second rotational positions of the wafer table. A wafer chuck desirably is mounted to the wafer table, in which instance the alignment feature can comprise an alignment mark located on at least one of the wafer table, wafer chuck, and substrate held by the wafer chuck. The wafer table comprises at least two movable mirrors including an X-direction movable mirror and a Y-direction movable mirror, and a respective interferometer is provided for each of the movable mirrors at each of the first and second rotational positions.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.